Communications networks employ satellites operating in geosynchronous orbits in combination with terrestrial facilities such as land lines, microwave repeaters, and undersea cables to provide communications over vast areas of the earth. Geosynchronous satellites and terrestrial facilities are both expensive to install and to maintain and thus are not a cost effective means of increasing network capacity. In addition, geosynchronous satellites which operate at an altitude of 22,300 miles above the earth are unsuitable for supporting cellular service because of the extremely high power levels that would be required to communicate with satellites at that altitude.
More recently, constellations of low earth orbit (LEO) satellites have been proposed and are being developed as a cost effective means for providing increased capacity and supporting cellular and broadband data service for communications networks. In such a constellation, the satellites are divided into a number of orbital planes. Because low earth orbit satellites move rapidly with respect to the earth, each orbital plane includes a number of satellites that maintain continuous coverage for underlying cells defined on the surface of the earth. The cells represent coverage regions for the satellites.
Low earth orbit satellites utilize antennas which form a cluster of beams matching the ground-based cells. In each satellite, the beams must be steered to maintain alignment with the cells during the time the satellite moves one cell width along its orbit. After the satellite has moved one cell width, all the beams are ratcheted forward one cell width in the direction of flight and the beams are reassigned to the next set of cells in the flight direction.
Existing beam steering systems are inadequate due to their size, complexity, and cost. Mechanical steering apparatuses, for example, are too bulky, heavy and/or consume too much power for use in satellites. Electronic steering systems typically use multiple phase shifters per antenna array element or a hybrid divider network with distributed phase shifters as a variable power divider network. The use of phase shifters greatly increases complexity of the antenna system and thus cost.